Vertical Beam Angle Control

Vertical Beam Angle Control – An advancement/requirement in modern Ion Implant Manufacturing

1 2 2 Christian Krueger , Robert Rathmell , Dennis Kamenitsa , and Bernhard Krimbacher3 1) Fab36, AMD, Dresden, Germany, 2) Axcelis Technologies Ltd., Beverly, Massachusetts, 3) Axcelis Technologies Ltd., Dresden Germany

Abstract. As the industry moves to the new technology nodes of 45 nm and 32 nm devices, implant angle control becomes even more crucial for consistent device performance. Commercial single wafer ion implanters are able to measure and correct the horizontal incident angle of the ion beam. But the vertical beam angle (VBA) control has become a very important parameter as well. In this work the authors demonstrate the impact of a tilt variation for a 65 nm and a 45 nm MOS transistor generated by different beam setups on one machine. Comparisons are made for each technology with a controlled angle variation of ±4°. The Vt-distribution should be reduced with better incident angle control allowing faster development of a new transistor node base line using the new VBA control technique from Axcelis. Keywords: Angle control, process control, threshold voltage PACS: 85.40.Ry, 81.05.Ea, 41.85.Qg

INTRODUCTION AND THEORY OF monitor and the surface of the wafer of about 30. The OPERATION VBA monitor is a faraday (2) with a thick slotted mask that limits the current transmitted to the collector as Manufacturing integrated devices at 45 nm and 32 the slots in the mask are tilted relative to the beam nm technologies and beyond becomes more angle [1]. This will produce a current as a function of complicated due to the tighter restrictions on transistor the tilt angle of the VBA monitor that peaks when the parameter variation. To fabricate a symmetric or an slots are aligned with the beam and that allows asymmetric transistor requires exacting process accurate determination of the “center of mass” or flux conditions and results for every manufactured wafer. weighted vertical angle of the beam. The fixed offset between the VBA monitor and the wafer surface is Tight control of threshold voltage (Vt) or Miller- calibrated with a sensitive channeling implant and Capacitance (CM) is needed to improve the performance of certain devices and achieve the allows one to set the chuck tilt relative to the actual transistor node requirements with stable results. The beam angle to the value specified in the recipe. new medium current implanter Optima MDTM The vertical beam angle is measured at the start manufactured by Axcelis ® is using a new technique of a processing job by tilting the chuck so that the for the vertical beam angle (VBA) measurement in VBA monitor is in line with the ion beam, as shown in order to compensate for a vertical beam angle variation Figure 1a. Data is collected at +/-5 about this point as from the expected or desired beam angle. The actual the chuck moves down and up through this range of vertical tilt angle between the plane of the scanned angles in about 4 seconds. This provides two sets of beam and the plane of the wafer is measured via a data that can be checked for consistency and beam profile mask attached to the linear portion of the dropouts. At these tilt angles, the wafer is safely mechanical scan arm supporting the wafer. Any offset above the ion beam. After the VBA measurement, the in the vertical beam angle is checked against the recipe chuck moves to the implant position, as shown in parameters and will be corrected automatically. Figure 1b, and passes through the scanned beam at the desired tilt angle relative to the measured beam angle, VBA HARDWARE while the VBA monitor remains fixed to the tilt mechanism and is safely out of the beam. The chuck and wafer are tilted about an axis parallel to the plane The vertical beam angle monitor, as shown in of the scanned beam in order to achieve the desired Figure 1, is rigidly mounted on the arm that supports tilt. the electrostatic chuck (1) such that there is a fixed offset between the angle of the surface of the VBA VBA variation per recipe 1

2 1 2 3 4 5 6 7 8 9 10 11 12 Recipe

FIGURE 2. VBA variation on one implanter for different recipes over a period of one month (Graph by AMD)

reduce the transistor parameter variation for a stable transistor node development. The horizontal beam angles are independently measured and automatically corrected at seven points across the wafer [1] to a recipe limit, which in this study was set to <0.5° (0.3° was typical).

ACTIVE INCIDENT ANGLE FIGURE 1. a) Scan arm at tilt angle to measure CORRECTION vertical beam angle of the accelerated ion beam. (1) electrostatic chuck with wafer (2) VBA-Faraday-Cup Active correction of the vertical tilt of the wafer by b) Scan arm at an implant position with VBA out of the beam (Figure by Axcelis) adjusting the wafer orientation about an axis parallel the scanned beam plane in order to compensate for the measured VBA offset could be one of the key factors STARTING CONDITIONS to get the transistor parameter distribution better under control. The new capabilities with the Optima MDTM Figure 2 shows a full range-median-average plot of are able to do this very well. Figure 3 shows a graph VBA measurements for 12 different recipes on one of for a number of different beam setups. The data show TM the Optima MD implant tools when the VBA was the measured Vertical Beam Angle and the measured, but not corrected. The actual VBA varies corresponding mechanical wafer tilt. The sum of these somewhat from setup to setup and the amount of is the actual angle between the beam and the surface of variation depends on the recipe conditions. Naturally, low energy recipes trend to show higher variations compared to recipes with higher energies due to a VBA active correction for P+ at 30 keV somewhat larger angle spread and magnification of 7.5 0 small angle errors in deceleration and focusing 7.4 -0.1 g g

7.3 e e

elements. -0.2 d

, 7.2 t A l The maximum variation for a 10 keV Boron (11) i -0.3 B

T 7.1 recipe is ~ 0.5° compared to a very low variation of V 7 -0.4 less than 0.1 from setup to setup for Arsenic (2 & 3) 6.9 -0.5 or Phosphorus (1 & 8) recipes with energies between 1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 50 keV and 250 keV. However, the data also show Batch that there can be over 1 variation from one recipe to another if no correction for VBA is used. In addition, Wafer Tilt Implant tilt VBA the setup for a given recipe may be different on every tool as well. Therefore, such factors could lead to FIGURE 3: VBA active correction for a 30 keV Phosphorus significant angle variation and make it very difficult to recipe with 7° Tilt requirement (Graph by Axcelis) the wafer and is shown to be very constant for all DEVICE DATA processing jobs. A separate paper at this conference uses a sensitive In order to determine the sensitivity of device channeling condition to show that the angle parameters, Vt and CM, to wafer tilt, a wafer split measurement of the Optima MDTM is repeatable experiment with a relatively high tilt angle variation relative to the planes of the crystal from setup to setup. was carried out using a Halo-and a Source Drain [2]. Extension, SDE-implant. The tilt of the wafer (the It was expected that the angle compensation would orientation of the wafer about an axis parallel to the reduce the transistor parameter distribution for a plane of the scanned beam) was purposely varied ± 4 certain device in a long term trend. for the Halo implant, compared to the Process of Record, POR. A separate implant using a +5 offset in the SDE tilt was done with the Halo implant at the normal tilt angle. The resultant device data show a

clear Vt-dependency on the incidence angle for the 65nm Halo implants. Surprisingly, variations of the tilt angle of the Halo implant influence the results much more than the Extension implant tilt variation. Figure 4 shows the influence the tilt angle changes for 65 nm and 45 nm technologies. The Vt-sensitivity of that particular NMOS transistor is different for each technology level, yielding 14 mV/deg and 21 mV/deg, respectively, with the higher sensitivity for the smaller node, as expected. Theoretically this effect would influence the transistor node development significantly if there are variations larger than ~0.5 at the 45 nm node and would make a clear interpretation of “controlled” experiments very difficult. In reality there is a relatively small reduction in the Vt parameter distribution on long term trends comparing data before active correction of VBA to those after active correction, as shown in Figure 5 for 4 months of data. The red bar shows when VBA auto- correction was turned on. That means that this parameter distribution is more influenced by other 45nm parameters like perhaps a variation in gate length, CM, or some other implant condition than by the implant incident angle itself. It is also possible that the implant angle was not ranging very far even before turning on active correction as was seen for some recipes in Figure 2.

FIGURE 4. Vt-sensitivity for a NMOS transistor at different technology levels (Graph by AMD) FIGURE 5: Long term Vt behavior prior to and after the use of the VBA unit to correct angles on all Optima MD (Graph by AMD) While the effect on the long term distribution is observed variation in device electrical parameters of relatively small, Figure 4 shows that there is a high real product so much suggests that either there was not sensitivity to beam angle for halo implants, and it is much angle variation in the critical implants prior to more and more important to minimize all relevant implementing active control or parameters other than sources of variation for an effective device beam implant angle are causing lot-to-lot variations development. that may be seen.

CONCLUSION REFERENCES The Optima MD Vertical Beam Angle measurement allows precise control of the orientation of the wafer 1. R.D. Rathmell, et al, “Implant Angle Control on Optima about an axis parallel to the plane of the scanned beam MD”, AIP Proc. of 16th Intl. Conf. on Ion Implantation during implant. Some devices studied show a high Tech., Marseille, France 349-352 (2006) sensitivity such that a variation of ~0.5 could result in significant changes in Vt. As expected, the sensitivity 2. R. D. Rathmell and Jonathan David, “Implant is increasing as the node size decreases. The fact that Angle Repeatability on Optima MD”, these controlling the VBA very tightly did not change the proceedings